1. Field of the Invention
The present invention relates to an image forming apparatus and method, and more particularly, to droplet ejection control technology suitable for reducing deterioration in image quality caused by ejection errors in nozzles of a recording head (which may also be referred to as a “print head”) having nozzle rows in which a plurality of liquid ejection ports (nozzles) are arranged.
2. Description of the Related Art
In an inkjet recording apparatus (printer), there is a problem in that the position of dots deposited on a recording medium may be displaced from their ideal positions (hereafter, this is referred to as “deposition position displacement”), due to causes such as variation in the ink ejection direction from the nozzles, displacement of the nozzle positions, displacement of the positions of the respective color heads, and so on., and consequently, print quality is impaired. In particular, when printing lines in graphs, figures or the like, or text, the decline in quality caused by displacement of the dot positions from ideal positions is particularly severe, and becomes a very important problem in terms of the quality of the printer (hereinafter, the quality of lines in graphs or figures, and text, is referred to as “line quality”).
The phenomenon of decline in line quality is now described with reference to FIGS. 17A and 17B, which show schematic drawings of situations in which an oblique row of dots (oblique line) is printed on a recording medium by ejecting ink from the nozzles in a line head. In FIGS. 17A and 17B, reference numeral 200 indicates a line head, reference numeral 202-i (i=1, 2, 3, 4, 5) indicates a nozzle, reference numeral 204-i indicates a dot deposited by a nozzle 202-i (i=1, 2, 3, 4, 5), and reference numeral 206-i indicates the center position of the dot. Furthermore, arrow A indicates the relative conveyance direction of the recording medium (for example, the recording paper) with respect to the line head 200.
FIG. 17A is a diagram in which an ideal oblique line is printed by ejecting ink normally from all of the five nozzles. Furthermore, FIG. 17B shows a row of dots formed in a case where the central nozzle 202-3 has produced an ejection error and the ejection direction is displaced toward the right.
As shown in FIG. 17B, when an oblique line is printed in a case where the ejection direction of the central nozzle 202-3 shifted to the right, if droplets are ejected under the same ejection control (ejection timings) as FIG. 17A, the dot 204-3 formed by the defective nozzle 202-3 is deposited in a position shifted to the right (FIG. 17B). Following the dot row (oblique line) in the line direction, a projection or depression is caused by the displaced dot 204-3. These projection and depression in the row of dots cause deterioration in line quality.
As described above, the depressions and projections in the dot row occurring as a result of deposition position displacement of the dots is a major cause of deterioration in line quality. Furthermore, in the inkjet recording apparatuses, decline in line quality is especially notable in the case of oblique lines such as that shown in FIGS. 17A and 17B.
In response to problems of deteriorated print quality due to deposition position displacement, technology has been proposed for preventing deposition position displacement by controlling the ejection timing from the respective nozzles (see Japanese Patent Application Publication Nos. 11-277733 and 2000-62148, for example).
Japanese Patent Application Publication No. 11-277733 discloses correction of positional displacement within the space of one dot, by dividing the ink ejection time for one dot into a plurality of time periods, and controlling the ink ejection timing between these divided times. On the other hand, Japanese Patent Application Publication No. 2000-62148 describes providing a device for delaying the ink ejection time in order to cancel out deposition position displacement in a line head.
Many of the technologies proposed conventionally in order to prevent deposition position displacement correct deposition position displacement in the main scanning direction (the shuttle movement direction) in the case of a shuttle scanning head, and in the sub-scanning direction (paper conveyance direction) in the case of a line head. In the case of deposition position displacement in these directions, the dot deposition positions are amended by controlling the ejection timing, and dot positions without any deposition position displacement (hereinafter referred to as “ideal positions”) are achieved.
However, in the case of deposition position displacement in a direction (hereinafter referred to as the “nozzle row direction”) which is perpendicular to the aforementioned direction, it is not possible to cause a dot to be deposited at the ideal position, even if the ejection timing is altered. With regard to this point, Japanese Patent Application Publication No. 2000-62148 points out the issue of deposition position displacement in the nozzle row direction, and states that “the ink ejection timings of the respective nozzles are delayed in such a manner that positional displacement is cancelled out”; however, Japanese Patent Application Publication No. 2000-62148 provides no concrete disclosure with regard to the method of resolving deposition position displacement.